Helicobacterpylori possesses two flagellin molecules, MA, the major species, and FlaB, which is expressed in minor amounts. This study sought to determine if one or both flagellin species are necessary for colonization or persistence by H. pylon. Thirty-six gnotobiotic piglets from six litters were given one of four isogenic strains of H. pylon orally. The bacterial strains used were strain N6, the wild type, which produced both FlaA and FlaB and was fully motile; N6flaB::km, which produced FlaA but not FlaB and was weakly motile; N6flaA::km, which expressed FlaB but not FlaA and was nonmotile; and N6flaA::cat/flaB::km, which produced neither flagellin and was nonmotile. Strain N6 colonized all piglets and persisted for 2, 4, and 10 days after inoculation. Both N6flaA::km and N6flaB::km colonized for 2 and 4 but not 10 days, and colonization was weak. N6flaA::cat/flaB:: km colonized for 2 days but did not persist for 4 or 10 days after inoculation. These findings demonstrate that both flagellin species are necessary for full colonization by H. pylon. Colonization for up to 4 days is possible in the absence of either flagellin species but not both.